Field of the Invention
This invention relates generally to thin-film phase-change memories, and in particular to a small phase-change switching volume formed by overlapping thing films.
Phase-change memory devices use phase-change materials, i.e., materials that may be electrically switched between a generally amorphous and a generally crystalline state, as an electronic memory. One type of memory element utilizes a phase-change material that may be, in one application, electrically switched between generally amorphous and generally crystalline local orders or between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states.
Typical materials suitable for such an application include various chalcogenide elements. The state of the phase-change materials is also non-volatile. When the memory is set in either a crystalline, semi-crystalline, amorphous, or semi-amorphous state representing a resistance or threshold voltage (Vt) value, that value is retained until reprogrammed, even if power is removed. This is because the programmed value represents a phase or physical state of the material (e.g., crystalline or amorphous).
Typically one of the limiting factors in the density with which such a non-volatile memory can be fabricated is not the size of the programmable phase-change element, but instead the size of the access transistor or other access device co-located with each phase-change memory element. This problem stems from the scaling of the maximum current supplied by the access device with its size, and thus memory element designs that can reduce the amount of current required for somewhat equivalently, the total power required) in order to switch the phase-change element are key for this technology, Particularly critical is the highest current (power) that is needed to melt the phase-change material during the programming of the high-resistance “RESET” state. In the RESET state, the current path through the phase-change element upon readout is forced to pass through some portion of the phase-change material that is in the amorphous phase, thus leading to high device resistance.
Two paths towards reducing this RESET current are to reduce the cross-sectional volume (or more appropriately, area) of the device that is switched between crystalline and amorphous, and to increase the thermal efficiency, so that most of the electrical power that is injected into the device goes towards melting the phase-change material. Key to this second point is the need to increase the thermal resistance between the switching volume and its surroundings. In particular, the electrodes that deliver current to the device need to have high thermal resistance yet low electrical resistance (because if they were highly resistive, then they themselves would heat up instead of the switching volume, thus requiring a larger total amount of power to be delivered from the access transistor or other access device). Because some current (power) would be “wasted” on heating the internal access electrodes, additional total current (power) would be required in order to successfully RESET the device by melting and quenching a sufficient portion of the phase-change material into the amorphous phase. Here the portion is sufficient if the electrical current passing through the element is forced to go through the amorphous phase of the material, which thus significantly increases the overall resistance of the element, representing a stored binary 1. If the cross-section of the electrical path in the vicinity of the switching junction is only 99% amorphous, then due to the high contrast between the resistivity of the amorphous and crystalline phases, most of the current will pass through the small remaining crystalline filament, thus leading to a lower overall device resistance and a perceived stored binary 0. Thus reducing the cross-section of the electrical path that needs to be blocked, ensuring that most of the electrical energy is injected as heat at this region and is thermally insulated from flowing elsewhere, and establishing a fabrication procedure which makes every memory element as identical as possible, are key goals.
Thus, there is a need for cell designs which combine small switching volume together with electrodes that have a high thermal resistance yet a low electrical resistance.